Recent years have seen explosive growth in the use of wireless communication systems. For example, wireless systems are commonly used in mobile telecommunication systems, such as cellular and personal communication systems (PCS), to provide voice communication, in wireless modems and network connections enable the transfer of data without a physical link between computers, and in satellite systems to transmit and receive audio and television programming.
A common feature of wireless communication systems is that a wireless device, a receiver or transceiver, often includes an apparatus or circuit for determining a quality of the received signal. Occasionally this determination of received signal quality is displayed in some manner to the user of the wireless device. One example is a bar graph on a mobile phone display that typically consists of four display characters or bars to indicate relative signal quality. In this type of display, the four bars are selectively activated or displayed in response to the internal determination of relative signal quality. None of the bars are activated if the received signal quality is unacceptable, one bar indicates a low quality signal relative to an optimal received signal quality, two bars a medium quality signal, and so on until all bars are activated indicating the optimal received signal quality.
In conventional wireless systems, the determination of relative signal quality is accomplished by simply measuring the strength of the received signal. The general understanding is that a strong received signal provides a higher quality of communication than a weak received signal. Thus, providing and displaying a received signal strength indication (RSSI) allows the user of the wireless device to determine the utility of the device for communication at any particular time. The strength of the received signal can be determined, for example, using the feedback signal of an automatic gain control (AGC) loop, or an output from a detecting logarithmic amplifier.
A graph of relative signal quality of a signal versus received signal strength for a conventional wireless communication system is shown in FIG. 1. Referring to FIG. 1, it is seen that a received signal strength of between about −110 and about −100 dBm yields a relative signal quality of between 0 and 0.2 times that of an optimal signal, and would be indicated to the user by activating none of the bars in the received signal quality display. A received signal strength of between about −100 and about −85 dBm yields a relative signal quality of between 0.2 and 0.4 times that of the optimal signal, and would be indicated to the user by activating a single bar in the received signal quality display. A received signal strength of between about −85 and about −75 dBm yields a relative signal quality of between 0.4 and 0.6 times that of an optimal signal, and would be indicated to the user by activating two bars. A received signal strength of between about −75 and about −60 dBm yields a relative signal quality of between 0.6 and 0.8 times that of the optimal signal, and is indicated to the user by activating three of the bars. A received signal strength of between about −60 and about −45 dBm yields a relative signal quality of between 0.8 and 1 times that of the optimal signal, and is indicated by activating all of the bars in the received signal quality display.
One shortcoming of the above approach, as discovered by the inventor of the present invention, is that RSSI is a poor indicator of the actual quality of communication available from a received radio signal.
In cellular applications, cellular operators wish to use power control on the base-station transmitters to minimize interference in the system. Reduced signal strength is likely to have no impact on actual communications quality (acutally enhancing overall signal/interference ratio—SIR—in the network). However, customers do notice that the RSSI has dropped, and mistakenly interpret this as a drop in network communications quality. If the visual indicator were truly indicative of actual communications quality (which would not have changed, or would have improved), this concern would be alleviated.
U.S. Pat. No. 5,289,178 discloses the display of a “raw” bit-error rate in a paging receiver. However, it does not suggest the display of “link classification” based on the effective (i.e., corrected) bit-error rate in a coded communications system such as a cellular telephone system.
Accordingly, there is a need for an apparatus and method for determining and indicating received signal quality that more accurately reflects the quality of communication available from a received signal. It is desirable that the apparatus and method be suitable for use with a broad range of wireless communication systems including telecommunication systems, wireless modems and satellite systems. It is also desirable that the apparatus and method is usable with existing infrastructure, such as telecommunication base stations, with little or no modification to the infrastructure. It is further desirable that the apparatus and method is compatible with display methods now used in wireless devices.